The invention relates to aluminum alloys which contain magnesium and silicon and articles extruded therefrom.
The aluminum-magnesium-silicon alloys as contemplated herein are alloys having a major content of aluminum and minor contents of magnesium and silicon, and are exemplified by known alloys identified by Aluminum Association designations in the 6000 series, e.g. alloys having aluminum association (AA) designations such as 6009, 6010, 6011, 6061 and 6063. One of the most widely used of these 6000 series alloys has been Alloy 6061.
These 6000 series alloys are heat treatable and are well known for their useful strength and toughness properties in both T4 and T6 tempers.
Typical 6000 series aluminum alloys are described in Park, U.S. Pat. No. 4,589,932, issued May 20, 1986. That patent describes alloys 6061 and 6063 in some detail and refers to alloy 6061 as being useful for sheet, plate and forging applications.
These alloys are further discussed in Jeffrey et al., U.S. Pat. No. 4,637,842, issued Jan. 20, 1987. In that case, a 6061 stock was used in producing aluminum sheet for the use in the manufacture of aluminum cans.
Another Alxe2x80x94Mgxe2x80x94Si alloy is described in Schwellinger et al., U.S. Pat. No. 4,525,326, issued Jun. 25, 1985. This alloy was designed for producing extrusions and contained as an essential component 0.05 to 0.20% vanadium.
With medium strength Alxe2x80x94Mgxe2x80x94Si alloys, maximum extrusion speed is controlled predominantly by the percentage of magnesium silicide in the alloy. This determines the hot flow stress of the alloy and therefore the temperature rise that occurs during deformation. The maximum extrusion speed is that at which the surface begins to tear or speed crack. This occurs when the surface temperature reaches the solidus temperature of the alloy. For any starting billet temperature, a reduction in the heat of deformation allows a higher speed.
The relation between temperature rise and speed follows a log (speed) relation. Therefore, small reductions in the magnesium silicide percentage and corresponding small changes in the flow stress and temperature rise can have a very significant effect on the maximum speed.
A typical AA6061 alloy in commercial use is 0.88% Mg, 0.60% Si, 0.20% Fe, 0.20% Cu, 0.08% Cr and less than 0.2% manganese and the balance essentially aluminum. Commercial operating conditions are generally non-optimum which results in incomplete solution treatment and in some instances precipitation of the magnesium silicide during quenching. It has been found that AA6061 is typically richer in magnesium and silicon than is actually required to achieve AA6061-T6 mechanical properties, which is the property target recognized for structural applications in the North American extrusion industry.
It is the object of the present invention to provide an alloy for structural applications having improved extrudability without risk of compromising mechanical properties.
The present invention in its broadest aspect relates to an extrudable aluminum based alloy consisting essentially of 0.60-0.84% by weight magnesium, 0.40-0.58% by weight silicon, 0.15-0.40% by weight copper, 0.06-0.20% by weight chromium, or 0.20-0.80% by weight manganese, less than 0.25% by weight iron, where Si greater than =(Mg/1.73+(Mn+Cr+Fe)/3xe2x88x920.04), and the balance essentially aluminum.
A preferred alloy contains 0.64-0.84% magnesium and 0.45-0.58% silicon, more preferably 0.64-0.80% magnesium and 0.45-0.58% silicon.
In the alloy of the present invention, the magnesium content has been reduced to the minimum possible for mechanical properties. In this way, the magnesium silicide content of the alloy has been reduced, providing a very beneficial effect on extrudability. Thus, there are productivity gains based on reduction in flow stress and extrusion pressure.
It has been found that it is the melting point of the alloys of this invention that is the direct cause of these alloys being capable of meeting AA6061-T6 mechanical properties with significantally improved extrudability